So, since I haven't seen it earlier in the thread, I will just put a bit of info out on the things that impact marksdwarf effectiveness. First off, there are three primary things that impact the effectiveness of a mele dwarf:
1. Skills such as weapon proficiency, dodger, shield user etc. Probably the most important for overall performance.
2. Basic attributes, such as strength, toughness, etc.
3. Weapons and armor, both quality and materials. Better gear significantly increases surviveability and effectiveness.
In terms of application mele dwarves are extremely simple. They path to the nearest enemy and get stuck in until somebody is dead. This means they are easy to use well since there is pretty much only one way to use them.
Now, in contrast using marksdwarves is a hell of a lot more complex. First off, there are additional factors that impact their battlefield effects. First off, the stuff that is the same:
1. Basic skills matter a LOT. However training them is harder since to raise marksdwarf skill consumes ammo and requires archery ranges which dwarves need to be carefully managed to use frequently.
2. Base attributes still help, though they matter less.
3. Crossbow quality. This makes bolts much more likely to hit. However it does NOT control the damage they do.
4. Bolt quality and material. This determines a bolt's effects on target. Bone and wood bolts, even masterwork, are a lot like nerf darts much of the time (enough of them are still quite dangerous though). Masterwork steel on the other hand will frequently penetrate and cause extensive internal injury.
5. Pathing to the target. Your marksdwarves dream of melee glory, as all Dwarves should. Don't give them the change or they will likely take advantage of it. Note that training your marksdwarves into legendary hammerdwarves then giving them copper crossbows and shields can still produce hilarious results though.
6. Visibility. Your dwarves have to be able to see the target. Fortifications can significantly complicate this. Cover and concealment (like bushes) may even matter, !!SCIENCE!! needs to be done there by somebody with more time than me. Best practice is to make hostiles advance over a paved lot just to be sure. Also this prevents spontaneous tree growth which most assuredly DOES screw up your engagement area.
If you can get all the above elements right, which is something I specialize in, marksdwarves are absolutely devastating. Legendary marksdwarves firing masterwork steel bolts routinely headshot goblins and forgotten beasts; watching a 80 goblin siege advance on a properly set up archery tower is like watching a movie of a World War I infantry assault. On the other hand getting everything right is hard and will never bee 100% reliable so have mele as a backup for when Murphy has his say in things.